Under-sea cables used for transmitting a large number of telephone channels are generally of the coaxial type and include a plurality of remote-fed amplifiers or "repeaters" placed at regular intervals in series in the cable to compensate for the attenuation of the signals therealong over the entire frequency band width used.
During the past few years cables of the "central strength member" type have been laid and have generally included a central conductor formed by steel strands contained in a copper tube formed by longitudinally welding a tape which has been rolled around the strands, with an insulator constituted by polyethylene extruded over the central conductor, and a return path formed by an aluminium strip that may optionally be laminated on one or both surfaces. Said aluminium strip is applied longitudinally over the insulator with its edges overlapping and a polyethylene sheath is extruded over the return path.
When laid in shallow water such cables have also had external armouring formed by one or more layers of steel wire helically wound around the cable.
Repeaters for connection to the cable have been fitted with a length of access cable at each end, said access cable being formed by a coaxial cable of smaller diameter than the line cable, with a copper central conductor and a copper braiding outer conductor which are insulated from each other by a concentric polyethylene layer. The method used up till now entails:
(a) interconnecting the central conductor of the line cable to the central conductor of the access cable. Said connection is made by brazing the central conductors to respective ends of a conical copper connection part, and is performed after a conductive sleeve part has been positioned over the insulation of the access cable. Said sleeve part is formed by a cylindrical aluminium body extended on either end by castellated tubes of decreasing length. The tube situated on the end nearest to the access cable is itself constituted by a copper portion welded to the aluminium portion which is integral with the cylindrical body and which has previously been surface treated with oxygen to facilitate subsequent adherence of a polyethylene layer which is surface moulded on these parts;
(b) surface moulding with polyethylene to reconstitute the insulation of the line cable and to seal it to the tube for access to the cylindrical body;
(c) welding the braiding of the access cable on the copper end of the tube for access to the cylindrical sleeve and surface moulding of a polyethylene sheath on the connection between the braiding and the tube;
(d) installing a cylindrical aluminium sleeve over the surface moulding of the insulator on the side nearest the line cable and spot welding this sleeve on one side to the return of the line cable and on the other side to the outer surface of the cylindrical body, this operation being followed by reconstitution of the outer sheath of the cable, and
(e) installing a protective neoprene nipple filled with polyisobutylene on the connection between the cable for access to the repeater and the tube, as described in U.S. Pat. No. 2,782,249 in the name of Paul Martin.
This method is very complex and very expensive; the connection obtained also had the serious drawback of creating a discontinuity in the characteristic impedance between the line cable and the access cable at the cylindrical sleeve body, which has the effect of reflecting echoes which are detrimental to proper high frequency transmission of the signals along the line.
A simpler solution has been proposed in U.S. Pat. No. 4,245,134 by International Standard Electric Co. In this solution, polyethylene is surface moulded in a frusto-conical configuration to reconstitute the insulation between the line cable and the access cable. A set of tubular parts in the form of cylinders and then in the form of truncated cones are welded to one another to provide electrical connection between the aluminium return path of the line cable and the copper braiding of the access cable. However, in order to link the aluminium conductor to the copper braiding, a zinc intermediate part must be used to prevent corrosion due to the difference in potential between the metals, and further, the welds are made over the polyethylene insulator in a zone where it is particularly thin and can therefore cause damage to the insulator. Lastly, although it is simpler than the previously described method, this method nevertheless requires a great number of operations.
Preferred implementations of the present invention simplify the operations necessary to make the connection while providing good continuity of impedance along the whole connection.
In the connection according to the invention, the conductive connection means for inter-connecting the return paths comprises an aluminium flared tube fitted tightly over the surface moulded insulation, with a layer of graft polyolefin or adhesive ionomer tape wound helically around the surface moulding being sandwiched in between the moulding and the flared tube, the wider end of the flared tube having a flared rim receiving and being welded to the return path of the line cable, and the narrower end of the flared tube being glued with conductive adhesive to the metal braiding of the access cable.
It also has preferably at least one of the following features:
The means for reconstituting the sheathing between the sheaths of the line cable and the access cable are constituted by a surface moulding made of thermoplastic material which is injected hot.
The means for reconstituting the sheathing between the sheaths of the line cable and the access cable include a surface moulding of thermoplastic material injected hot and extending from the outer sheath of the line cable to the vicinity of the conical portion of the flared tube, said surface moulding ending in a splined portion fitted with an elastomer nipple filled with polyisobutylene.
The outer conductor of the access cable is surrounded by an elastomer sleeve which is connected to said elastomer nipple.
In the method of manufacturing a sealed connection
(a) the insulation of the line cable and that of the access cable are removed from the zone required for connection by cutting them obliquely; PA0 (b) the end of the metal braiding which forms the return path of the access cable is rolled towards the repeater; PA0 (c) the axial conductor of the access cable is brazed to the small end of the conical conductive metal connection part and the axial conductor of the line cable is brazed to the large end of the conical conductive metal connection part, PA0 (d) a layer of insulating thermoplastic material is surface-moulded over the conical connection part and extends between the insulation of the access cable and of the line cable; PA0 (e) at least one graft polyolefin or adhesive ionomer tape is helically wound around said layer of insulating thermoplastic material; PA0 (f) a conductive aluminium flared tube having a flared rim at its larger end is fitted over said layer which is surrounded by said tape; PA0 (g) said flared rim is welded onto the end of the return path of the line cable; PA0 (h) the end of the metal braiding which forms the return path of the access cable is turned down over the smaller end of the flared tube and is glued thereon with a conductive adhesive; and PA0 (i) a sheath is surface-moulded over at least the end of said flared tube nearest the line cable, said surface moulding being performed at a sufficiently high temperature to melt the graft polyolefin or adhesive ionomer tape, the connection between said sheath and the sheath of the access cable thereby being completed.
It further has preferably at least one of the following features:
The sheath of the line cable is connected to the sheath of the access cable entirely by surface moulding.
The end of the flared tube nearest to the line cable is covered by surface moulding, imparting a splined configuration to the end of surface moulding, and an elastomer connection nipple is fitted on the end of the sheath of the access cable on said splined end.
The end of the return path of the access cable is surrounded with an elastomer sleeve and the corresponding end of said nipple is fitted onto the end of said sleeve.
The mould which is suitable for use in the method of surface moulding over the conical connection part and between the insulation of the access cable and the line cable has two half shells each of which has an inner part made of metal which is a good conductor of heat and a steel outer part, or else is entirely made of a metal which is a good conductor of heat and has at least one orifice for injecting thermoplastic material into the smallest diameter zone at the end nearest the access cable.
It further has preferably at least one of the following features:
It includes two insertion ducts in the cavity of the mould, said ducts being disposed symmetrically relative to the flash line of the two half shells and communicating with the injection orifice.
It includes grooves in the half shells on either side of the moulding cavity, the depth and width of these grooves being sufficient to reduce considerably the transmission of heat to the insulation of the access cable and of the line cable, said grooves being located both in the inner parts made of said metal which is a good conductor of heat and in the outer parts.
It includes means for maintaining the inner conductor of the access cable under mechanical tension during moulding.
Said means include a clamp to grip the access cable, a ring which forms a stop and a spring housed in a bore of a part disposed in the end of the inner duct of the mould.